Tuning fork



Feb. 14, 19250 J, R SHONNARD 2,497,143

TUNING FORK Filed Oct. 25, 1946 IN VEN TGR.

Patented Feb. 14, 1950 49"?43 UNITED STATES PATENT OFFICE TUNING FORK John R. Shonnard, New York, N. Y., assignor to Times Facsimile Corporation, New York, N. Y., a corporation of New York Application October 23, 1946, Serial No. 705,021

8 Claims. (Cl. 84-457) l 2. This invention relates to mechanical resonators smaller fork to longitudinal stresses. It is found and more particularly tuning forks and the like. that a fork constructed in this manner is stabi- For many applications of tuning forks it is lized over a wider range of temperatures than important to stabilize with a high degree of prein the case of laminated forks employing abutcision, the pitch or vibration frequency thereof ting laminations of substantially identical outunder normal conditions of use. In order to line or configuration secured over their entire avoid changes in the vibration frequency with areas. changes in ambient temperature, it has been pro- The dimensions of the tines depend upon the posed to construct a tuning fork of laminations desired frequency of vibration and the ratio beof different metals, the proportions of the metals 10 tween the vibratory portions of the respective and their elastic properties being so related as fork elements depends upon the temperature to provide a substantially constant vibration frecoefficients of expansion and elasticity of the quency over a considerable range of ambient metals employed. Thus, in the case of the comtemperatures. In one fork of this type, similar monly used carbon steel and nickel-alloy steel bifurcated laminations of carbon steel and such as Invar or Elinvar, the proportion of carnickel-alloy steel are welded or brazed together bon steel to nickel-alloy steel is approximately over their entire opposed faces. in the ratio of one to ten. The cross-section of In general terms, the object of the present the tines of each fork may vary from a square invention is to provide an improved resonator to a rectangular shape but the thickness of each or tuning fork of this character which is easier fork should be sufficiently great so that the tines to adjust and assemble, and which can be accuare stiff in the direction transverse to normal rately compensated to provide a stabilized frevibration, and neither fork tends to vibrate as a quency characteristic over a wide temperature reed. The forks are attached together by solderrange. ing or welding, or by the use of screws or rivets In accordance with the invention, the bimetalin such a manner that the contact area is relic temperature-compensated fork is preferably stricted lengthwise of the fork. Therefore the constructed of two superposed balanced fork two fork elements are free to expand longituunits, the smaller of which is mounted with its dinally without imparting vundesired stresses to tines attached to the tines of the other element. either fork. When constructed in this manner, In this manner the two forks are constrained to as stated above, it is found that the vibration vibrate as a unit, the resonant frequency of the frequency of the fork remains constant over a composite fork being determined by the dimenwide range of temperatures.

sions and elastic properties of both of its com- Other objects and advantages of the invention ponent elements. Since the forks may be sepawill appear from the following description of the rately balanced before mounting one upon the preferred embodiments thereof shown in the acother, balancing the combined fork units is praccompanying drawings, wherein tically unnecessary after assembly. After secur- Fig. 1 is a plan view of a fork embodying the ing the fork units together in any suitable maninvention;

ner. the frequency is adjusted by trimming 0r Figs. 2 and 3 are end and sectional views of the grinding the tines. fork shown in Fig. 1, Fig. 3 being taken along the Since one of the laminat-ions is relatively thin section line III-III of Fig. 1;

in the prior construction, it has been deemed Figs. 4 and 5 are sectional views similar to Fig.

necessary to solder or weld the fork laminations 3 showing two modifications of the invention; and

over their entire areas in order to secure unvary- Fig. 6 is a diagrammatic view of a constant ing permanent characteristics and avoid spurious frequency oscillator utilizing the improved tuning vibrations of the thinner lamination. In accordfork.

ance with a further feature of the invention, the Referring to Figs. 1 to 3, a tuning fork is shown smaller element, for example of carbon steel, is comprising a main body portion or fork element made short and thick enough to impart the necesiii consisting of a bifurcated metallic bar atsary stiffness to such element and it is then setached to a suitable mount or support li, as by cured preferably at or near the tip ends of the means of a bolt l2. The fork lil includes two tines only, to the other main element of the fork. symmetrical parallel tines as in the conventional In this manner the main body portion of the tuning fork. In accordance with the invention, tuning fork is free to expand and contract witha compensatory metal plate in the form of a out subjecting. the compensatory element orsecond fork l5 is mounted uponthe first fork element I0. The second fork element I5 is rigidly attached to the element ID, as will be described hereinafter, so that the two fork elements vibrate as a unit at a single resonant frequency determined by the sizes and elastic properties of the vibratory portions of both forks. Thus, for example, in order to secure stabilized frequency characteristics over a wide range of ambient temperatures,l the fork I may be constructed of nickel-alloy steel such as Invar or Elinvar, and the fork I of ordinary carbon steel. Since the nickel-alloy steel has a positive temperature coefficient of elasticity and the carbon steel has a negative temperature coeicient of elasticity, by controlling the ratio of the masses of the tines of the two forks, the normal frequency change of each fork resulting from changes in size and elasticity with changes in temperature may be compensated so that the composite fork has a fixed frequency of vibration over a wide temperature range.

As shown, the carbon steel fork I5 is provided with small :bosses or projections preferably but not necessarily at the ends of the tines to form a contact area IB of limited size between the two forks I0 and I5. The forks may be attached at the point I6 in any suitable manner, as by soldering, brazing or welding. If each of the forks is dynamically balanced and ground to the proper dimensions before assembly, the final frequency adjustment may be made readily by grinding the edges of one or both of the forks without disturbing the dynamic balance of the composite unit. The point of attachment may be at the ends of one or both forks, or along the lengths of the tines of both forks.

If desired, more than one compensating fork may be mounted on the main fork I0. Thus, as shown in Fig. 4, two compensating forks I1 and I 8 are mounted on opposite sides of the main fork I0, each of the compensating fork elements being similar to that shown at I5 in Fig. 3. In case the masses of the compensating forks are so small that the physical dimensions are reduced much below that shown in the drawings, it is necessary to guard against the possibility of transverse vibrations of the compensating elements. This is done by keeping the thickness of the compensating elements sufficient to provide substantial stiffness against transverse vibrations. If the size of the compensatory unit is sufficiently great, the length of the tines may be increased over that shown in Figs. 3 and 4.

This modification is shown in Fig. 5 wherein the two forks are attached together at tip ends ing tines as long as the fork I0. In this instance, the forks are secured together by clamping screws 2I, the ends of the tines being slotted to permit lengthwise adjustment of the compensatory fork unit, if desired. While in each modication the two forks are shown attached together in such relation that the throats of the tines are in alignment, this relation may be varied somewhat so long as the fork units are attached `together so that they vibrate at a single resonant frequency.

In actual practice, a fork of the character described is normally maintained in continuous vibration by electromagnetic or other conventional means. Thus, as shown in Fig. 6, the fork I0 may be disposed between pickup and drive magnets 22 and 23 respectively, adapted to maintain the fork in continuous vibration. The terminals of the pickup magnet 22 are shown as connected to the input circuit of an amplifier 24. The outof the tines, the compensatory fork unit havput circuit of the amplifier 24 is connected to supply a constant level drive current of a frequency determined by the vibration frequency of the fork to the drive magnet 23, thus maintaining the mechanical resonator in continuous operation. The amplifier 24 may also be employed to furnish a current of constant frequency for driving facsimile equipment, timing mechanisms and other similar devices.

While I have shown several embodiments of the invention in order to explain the principles thereof, it will be understood that the invention is not limited to these specific examples. Other modifications in the sizes and arrangement of the fork elements will occur to those skilled in the art and may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

1. A tuning fork comprising a plurality of superposed bifurcated elements rigidly attached together to vibrate as a single unit at a preden termined vibration frequency, the areas of contact between said elements being confined to a portion of the length of each element which is a small fraction of the total length thereof.

2, A tuning fork comprisingv a main bifurcated element and a second similar bifurcated element which is superposed upon and substantially shorter than the main element, each of said elements having two parallel tines of equal mass, the tines of both elements being rigidly fastened together to insure vibration of both parts of the fork as a unit and the oposing faces of the two elements being spaced apart except at the ends of the tines of said second shorter element.

3. A tuning fork comprising two superposed bifurcated elements of unequal length rigidly attached together to vibrate as a unit, each of said elements having a pair of symmetrical tines fastened to the tines of the other element and arranged with the throats in substantial alinement, the attachment of the shorter of said elements being confined to the tip ends of its tines to minimize internal stresses from longitudinal 1 expansion of said elements.

4. A tuning fork comprising a main body portion having opposed tines, and a superposed cornpensatory metallic element of similar configura-y tion mounted upon and rigidly fastened at the tip ends of its tines to the tines of the main body portion, the body portion and compensatory element being unattached at the heel ends opposite the tines to minimize internal stresses resulting from unequal expansion thereof with varying temperatures.

5. A tuning fork comprising a main metallic body portion having opposed tines, and a bifurcated compensatory metal plate superposed thereon and rigidly attached at the forked end thereof to the tines of the main body portion whereby the said body portion and metal plate vibrate as a unit and the fork frequency is determined by the elastic properties of both of said components, the point of attachment of said compensatory metal plate to said main body portion being restricted to a 'lengthwise contact area which is a small fraction of the length of said plate.

6. In combination, two metallic forked elements of different physical characteristics rigidly attached together in superposed relation to vibrate as a tuning fork at a single vibration frequency, said forked elements being attached only at the tines.

76 7. In combination, a tuning fork composed of a metal having a positive temperature coefficient of elasticity and a second tuning fork composed of a metal having a negative temperature coeflicient of elasticity, said forks being rigidly attached to each other in superposed relation to vibrate as a unit, the contact area between said forks being relatively short along the length of either of said forks to minimize internal stresses therein resulting from unequal expansion of the forks upon changes in ambient temperature.

8. In a mechanical resonator of the tuning fork type, two forks of metals having dilferent elastic properties, the tines of said forks being rigidly attached together in superposed relation to vibrate at a single resonant frequency, said forks being unattached except at the tines so as to be independently expansible lengthwise with changes in ambient temperature to obviate stressing of the metal resulting from unequal expansion.

JOHN R. SHONNARD.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,653,794 Whitehorn Dec. 27, 1927 1,715,324 Haglund May 28, 1929 15 1,880,923 Eisenhour Oct. 4, 1932 

